1. Field of the Invention
This invention relates to electrophotographic recording apparatus for facsimile devices which are capable of recording received image data with high quality without using an image data memory such as a page memory.
2. Description of the Prior Art
Carlson's process is typical of electrophotographic recording systems using toner. .This process includes charging, exposure, development, transfer, cleaning and erasing steps performed sequentially separately and the respective components of the system used are disposed dispersively around a photosensitive body.
FIG. 6 shows time changes in the surface potential of the photosensitive body from charging to exposure in Carlson's process. As shown in FIG. 6, the surface potential V caused by a charger attenuates with time, which is hereinafter referred to as a dark attenuation. Only that portion of a photosemiconductor layer of the photosensitive body exposed by light irradiation means becomes conductive and electrons become freely movable therein and thus the potential at the surface of that portion irradiated with light rapidly decreases. At the exposure step, an electrostatic latent image is formed on the photosensitive body surface as the difference between the quantity of electric charges in the areas where light strikes and the quantity of electric charges in the areas where light does not strike. The dark attenuation rate is, for example, about 20% in 5 seconds. The potential of electrostatic charges formed on the photosensitive body depends greatly on the time required for the charging and the time taken from the end of the charging to the start of the exposure. Therefore, in order to provide a high quality image without uneven density while maintaining the surface potential constant, it is necessary to maintain constant the time taken from the charging to the development.
The following problems arise in the use, in a facsimile device, of an electrophotographic recording apparatus according to Carlson's process and having the above qualities. Most of facsimile devices employ a redundency reduction encoding system which compresses and transmits image data in each line in accordance with the degree of succession of white pixels and dark pixels in order to shorten the transmission time and the length of data contained in each line differs from that in a different line.
Therefore, the time required for decoding at the receiver end varies from line to line. Thus, when such image data is converted to an optical signal at the exposure unit without storing such data in memory, the time taken from the charging to the end of the exposure varies line to line and as a result the attenuation of the surface potential at the photosensitive body due to dark attenuation varies from line to line, the quantity of toner deposited at the subsequent exposing step varies from line to line to thereby produce variegated density distribution.
Therefore, if an electrophotographic recording apparatus using Carlson's process is applied in a facsimile device, a memory having a capacity large enough to store at least one page of image data must be provided, the image data stored in the memory is delivered to the exposure means at constant timing, the time intervals taken for charging, development and transfer of data in all the lines must be equal. The provision of the memory increases the whole system cost.
In contrast with the recording apparatus using Carlson's process, Published Examined Japanese Patent application No. 59592/1985 discloses a recording apparatus in which a photosensitive body includes a three-layered structure which comprises a transparent support layer, a transparent conductive layer and a photosemiconductor layer. An exposure device and a developing unit are disposed inside and outside, respectively, of the photosensitive body in opposing relationship such that simultaneously with light irradiation from the inside of the photosensitive body, toner is fed from the outside of the photosensitive body, which removes the use of a charger. According to this techniques, charges are produced at that portion of the photosensitive body irradiated with light, and toner having a polarity opposite to that of the charges is deposited on that portion to form a toner image.
According to this technique, an electrostatic image is formed and developed at a single position around the photosensitive body, so that the image is not influenced by dark attenuation, which is prospective as the electrophotographic recording system used in a facsimile device.